Post-auricular Versus Frontal Bispectral Index

Electroencephalogram (EEG) derived monitors during practice of general anesthesia; allow the titration and maintenance of an adequate depth of anesthesia, advantages from reducing the recovery time after waking, as well as the risk of anesthetics adverse events . There are various types of EEG-derived monitoring devices that are used to monitor the depth of anesthesia, and among the established devices is bispectral index (BIS) monitor. It is a quantitative electroencephalographic device that is widely used to assess the hypnotic component of anesthesia, and a level between 40 and 60 is recommended for an adequate level of the hypnotic state. However, the use of BIS in certain surgeries is challenging because of the proximity of the forehead sensor to the surgical site. There are high possibilities of interruption of BIS recording due to contamination of the forehead sensor with blood or antiseptic cleaning solution. At the same time, the design and size of a BIS forehead sensor in the form of a long strip can also interfere with the site of surgical incision. Several alternative BIS sensor placements have been studied for cases in which the frontal setup is not feasible. However, few studies studied the placement of the BIS sensor at the post-auricular area as an alternative method of monitoring the depth of anesthesia.

Twelve Patients age ≥18 years, ASA I-III patients scheduled for elective renal surgeries. Monitoring of the depth of anesthesia will be performed using an Infinity BISx SmartPod® (Aspect Medical Systems, Newton, MA, USA) and an Infinity® Delta XL monitor (Dräger Medical, Lübeck, Germany). Two standard BISx Quatro® Sensors (Aspect Medical Systems) were placed at the forehead and post-auricular area. Before the induction of anesthesia, 2 BIS sensors (BISTM Quatro Sensors, Aspect Medical Systems, Newton, MA, USA) will be applied to each patient, 1 across the forehead and 1 along the post auricular area and attached each sensor to each BIS monitor (BIS-VistaTM monitors, Aspect Medical Systems, Newton, MA, USA). During induction, patients will be instructed to keep their faces relaxed (eyes closed, mouth closed, no facial expressions). The skin on the forehead will be cleaned with an alcohol 70% swab, and 2 to 5 seconds of digital pressure applied over the sensor leads. The sensor is comprised of disposable wet gel electrodes. Electromyographic activity of the frontalis muscle is measured by lead 4, which is the ground electrode as well. Frontal sensors will be applied with lead 1 at the center of the forehead; lead 2, 2.8 cm lateral to lead 1; and lead 3 at the temporal area between the lateral canthus and the hairline. Post auriclar sensors will be applied to the same side of the face, with lead 1 approximately 2.5 cm medial to the mastoid area, post-auricularly next to the hairline. Lead 2 will be attached at the mastoid area and lead 3 will be attached on the same side at the temporal area between the lateral canthus and the hairline. The titration of anesthetic depth will be maintained at BIS values between 40-60. Sudden, high EMG scores were identified as artifacts and the associated BIS values were eliminated in data analysis. Measurements: From each BIS monitor, the investigators will collect 3 pieces of data at each of 6 times points: BIS score, signal quality index (SQI) score determining the strength of the signal, and electromyography (EMG) score: before the induction of anesthesia (awake), at loss of the eyelash reflex (LOC), after intubation (intubation), after the first surgical incision (incision), every 30 minutes during the intraoperative period (maintenance), and at spontaneous eye opening upon emergence from anesthesia (emergence).